The present invention relates to an electric double layer capacitor (EDLC) and, more particularly, to a carbon material for EDLCs, a method of producing a carbon material for EDLCs, an electric double layer capacitor (EDLC) and a method of fabricating such an EDLC.
An EDLC, also known as a supercapacitor or electrochemical capacitor, has a pair of polarized plates made of activated carbon. These plates are placed opposite to each other via a separator and are used as a positive electrode and a negative electrode, respectively. Each plate is impregnated with an aqueous electrolyte solution or a nonaqueous electrolyte solution and contacted with a collector. Each polarized plate is manufactured, for example, by (1) adding small amounts of a conductive additive and binder to activated carbon, kneading them together and rolling the mixture into a desired form, (2) adding small amounts of a conductive additive and binder to activated carbon, dissolving the mixture to form a slurry and applying it to a collector, or (3) mixing a small amount of uncarbonized resin into activated carbon and sintering the mixture.
In the past, it has been considered that the capacitance of an EDLC is roughly in proportion to the surface area of the polarized plates. For this reason, activated carbon with large specific surface area has been used as a carbon material for EDLCs.
Activated carbon is prepared by carbonizing a carbonaceous material at a temperature lower than 800xc2x0 C. and then activating the material. The activation is performed either by heating at 600-1000xc2x0 C. in an ambient of steam or carbon dioxide or by adding zinc chloride, potassium hydroxide, or the like and carrying out a heating step in an inert ambient. During this activation process, a large number of micropores adapted for adsorption are formed in the surface of the carbon material produced by the carbonization process.
In the past, in order to make the capacitance of the EDLC as large as possible, attempts have been made to increase the surface area of activated carbon. For example, Japanese Patent Laid-Open No. 78513/1988 proposes a technique for obtaining activated carbon by mixing potassium hydroxide into petroleum cokes and calcinating the mixture. It is said that the specific surface area which was about 1500 m2/g at maximum in the past could be increased to 2000-3500 m2/g. However, we have found that even if activated carbon whose specific surface area is increased is used as a polarized electrode material, limitations are imposed on the obtainable capacitance since the surface area per unit volume decreases by strong activation.
The present invention is intended to provide an EDLC having a large capacitance.
It is a specific object of the invention to provide a carbon material from which an EDLC having a larger capacitance than heretofore can be fabricated.
It is another object of the invention to provide a method of producing this carbon material.
It is a further object of the invention to provide an EDLC using this carbon material.
It is yet another object of the invention to provide a method of fabricating this EDLC.
A carbon material produced in accordance with the present invention and used for an EDLC has crystallites of graphite-like carbon produced by activating a carbon material. The carbon material is characterized in that the crystallites have interlayer distances of 0.365 to 0.385 nm.
A method of fabricating a carbon material for an EDLC in accordance with the present invention starts with heating a raw material to carbonize it, thus growing crystallites of graphite-like carbon. The heat-treated carbon material is activated with an alkali. The obtained carbon material consists of crystallites of carbon having interlayer distances of 0.365 to 0.385 nm.
In a method of producing a carbon material for an EDLC in accordance with the present invention, the carbon material is activated with steam. A carbon material consisting of crystallites of graphite-like carbon is obtained. These crystallites have interlayer distances of 0.365 to 0.385 nm.
An EDLC in accordance with the present invention has polarized plates immersed in an organic electrolyte. These polarized plates are made of a carbon material including crystallites of graphite-like carbon produced by activating a carbon material. The crystallites have interlayer distances of 0.365 to 0.385 nm.
A method of fabricating an EDLC having polarized plates immersed in an organic electrolyte in accordance with the present invention uses a carbon material having crystallites of graphite-like carbon produced by activating a carbon material. The crystallites have interlayer distances of 0.365 to 0.385 nm. After the EDLC has been assembled, a voltage in excess of the rated voltage is applied across the polarized plates to induce a capacitance.
The EDLC in accordance with the present invention has the polarized plates consisting of a carbonaceous material that expands on application of the voltage. The polarized plates are held within a dimension-limiting structure that limits the expansion on application of the voltage.
Other objects and features of the invention will appear in the course of the description thereof which follows.